In a typical frequency synthesizer, such as the circuit illustrated in FIG. 1, the output frequency of a reference oscillator 20 is calibrated or tuned to a predetermined value. A desired output frequency is then produced by adjusting the "N" value of the divide-by-N divider 13 in the synthesizer loop 11. The N value can be mathematically determined simply since the reference oscillator frequency is predetermined. When utilizing this approach, it is necessary that the crystal oscillator 20 be capable of being tuned to this predetermined value. Crystals are subject to variations in tolerance due to manufacturing processes, as well as aging and temperature effects. For example, a reference crystal with a nominal frequency of 10.240 MHz can have a minimum pullability or adjustment of 15 parts per million (PPM) per picofarad (pF). This translates to a maximum of 22 KHz at a transmit frequency of 433.92 MHz when a tuning varactor is moved 3.4 PF. If the synthesizer is used in a transmitter that is frequency modulated, the varactor 26 can be modulated by varying the voltage applied to the varactor 26, thereby providing a varying capacitance and the FM modulation. At 423.225 MHz, the local oscillator (LO) frequency for the receiver, the pullability is 21.6 KHz. This would make the tolerance of the crystal 20 PPM, which combined with a 10 PPM temperature tolerance and 23 PPM aging tolerance totals 53 PPM or 24 KHz at 433.92 MHz. In this application, a tighter crystal specification would be needed (at a higher cost), if the reference oscillator were tuned by the varactor only. A tuning approach that permits the use of a less precise and less expensive crystal while permitting tuning operation of the frequency synthesizer to produce a desired output would be desirable.